Mike,
I don't know an auction house, but I do frequent DJ's Loan up in Bothell. I've known them to be honest in my dealings and keep their word to an extreme for my dealings. They deal in a fair number of doubles and vintage guns. My purchase was a nice M12 20ga 28" solid rib. They held that gun for about 6 months for me just on my word while ATF took their time to renew my C&R license.